A system has been put into practical use wherein servers or disk drives are connected by fiber channels, via a broad-band network, to create data backups in remote areas. According to this system, since data backups can be stored in remote locations that are less affected by natural disasters occurring at local areas, the possibility that data will be lost is extremely low. In this system, the bandwidth of the broad-band network is smaller than the bandwidth of a local network, and congestion may occur at a switching device that serves as a relay for the broadband network and the local network.
Conventionally, to prevent such congestion, a switching device is used that employs buffer credit for the performance of flow control (see, for example, Japanese Patent Publication number 2000224180A). According to buffer credit control, a notification is transmitted to a disk drive connected to the switching device concerning the remaining capacity of a communication data buffer provided for the switching device, and upon receiving this notification, the disk drive adjusts the transmission of frames so as not to exceed the number of credits. Thus, buffer overflow at the switching device can be prevented.